transfer them to the ground.
The suitability of
various foundation systems and allowable values for
systems are illustrated in Figure 2-2.
their design must be determined from available data
or by a program of soil borings and laboratory tests.
(a) In buildings where a moment-resisting
frame resists the earthquake forces, the columns and
beams act in bending (a of Figure 2-2). During a
large earthquake, story-to-story deformation (story
A building is not merely a
drift) may be a matter of inches, without causing
summation of parts (walls, columns, trusses, and
failure of columns or beams. The drift, however,
similar components), but is a completely integrated
may be sufficient to damage elements that are rigidly
system or unit that has its own properties with
tied to the structural system, such as brittle
respect to lateral-force response. The designer must
partitions, stairways, plumbing, exterior walls, and
trace the forces through the structure into the
other elements that extend between floors. For this
ground, and make sure that every connection along
reason, buildings can have substantial interior and
the path of stress is adequate to maintain the
exterior nonstructural damage, possibly approaching
integrity of the system. It is necessary to visualize
50 percent of the total building value, and still be
the response of the complete structure, and to keep in
considered structurally safe.
Moment frames are
mind that the real forces involved are not static, but
desirable architecturally because they are relatively
dynamic; are usually erratically cyclic and repetitive;
unobtrusive compared with shear walls or braced
may be significantly larger than the design forces;
frames, but they may be a poor economic risk unless
and can cause deformations well beyond those
special damage control measures are taken.
determined from the design forces.
(b) Buildings with shear walls (b of Figure
(2) Lateral force system types. Over a dozen
2-2) are usually rigid compared with buildings with
With low design stress
described in Chapter 7. All of the vertical elements
limits in shear walls, deformation due to shear forces
of these lateral-force systems consist of: (a) moment-
(for low buildings) is negligible.
resisting frames within a three-dimensional space
construction is an excellent method of bracing
frame system; (b) a coordinated system of shear
walls; (c) a three-dimensional system of braced
components, but architectural considerations may
frames; or (d) a combination or "dual system" of
limit its applicability.
Shear walls are usually of
moment-resisting frames with either shear walls or
reinforced unit masonry or reinforced concrete, but
braced frames. These vertical elements may be used
may be of wood in wood-frame buildings up to and
including three stories. Shear wall design is
described herein. All of the horizontal elements of
relatively simple, except when the height-to-width
ratio of a
diaphragms or horizontal bracing systems.